The invention relates to a method for fractionating a feed gas containing hydrogen and carbon dioxide, from which feed gas carbon dioxide is largely selectively removed by scrubbing in a first gas scrubber using a sulphur-free scrubbing agent, wherein a scrubbing agent stream laden with carbon dioxide and co-absorbed hydrogen is obtained which is subsequently expanded in an expansion vessel in order to convert co-absorbed hydrogen into the gas phase.
Gas scrubbers are used in order to remove specific components from gas mixtures with the assistance of a liquid scrubbing agent in order thereby to produce a product gas. They exploit the characteristic of liquids of absorbing gaseous substances and keeping them chemically or physically bound in solution. The effectiveness with which a gas is absorbed by a liquid is expressed by the solubility coefficient: the better the gas dissolves in the liquid, the greater is its solubility coefficient. The solubility coefficient generally increases with failing temperature and rising pressure.
Subsequent to gas scrubbing, the scrubbed gas components are removed from the liquid used as the scrubbing agent, whereby the scrubbing agent is regenerated. While the scrubbed gas components are either discarded or put to economically viable use, the regenerated scrubbing agent is normally reused in the gas scrubber.
According to the prior art, hydrogen is obtained on a large industrial scale by converting feed materials containing carbon into a crude synthesis gas by gasification. Such a crude synthesis gas also contains, in addition to the wanted hydrogen, carbon monoxide and a series of other unwanted constituents, such as carbon dioxide (CO2), hydrogen sulphide (H2S) and carbonyl sulphide (COS). After at least partial conversion of the carbon monoxide with water to yield hydrogen and carbon dioxide, the crude synthesis gas is subjected to preferably physical gas scrubbing in order to separate the unwanted constituents. Such a method is appropriate for this purpose since crude synthesis gas is today generally produced at elevated pressure, and the efficiency of a physical gas scrubber increases in a first approximation linearly with the operating pressure. Methanol scrubbing, in which liquid methanol is used as the scrubbing agent at temperatures far below 0° C., is of particular significance for the purification of crude synthesis gases. “Gas Separation & Purification”, December 1988, vol. 2, p. 171-176 describes a methanol scrubber in which carbon dioxide and sulphur components are selectively removed in two successive scrubbing steps from a crude synthesis gas containing hydrogen, carbon dioxide and H2S and COS. To this end, the crude synthesis gas is passed from the bottom upwards through an absorber column, in which a first and a second scrubbing section are arranged one above the other. The carbon dioxide is separated by using unladen methanol in the upper, second scrubbing section, while the sulphur components are removed in the first scrubbing section by scrubbing with a proportion of the methanol which has already been laden with carbon dioxide during separation of the carbon dioxide. Since the sulphur components have a solubility coefficient with regard to methanol which is a multiple higher than that of carbon dioxide, only a fraction of the quantity of scrubbing agent laden in the second scrubbing section with carbon dioxide is required to separate them.
Although hydrogen and carbon monoxide have a solubility coefficient with regard to methanol which is several orders of magnitude lower than carbon dioxide or the sulphur components present in the crude synthesis gas, small quantities of hydrogen and carbon monoxide are inevitably also dissolved by the methanol scrubbing agent during scrubbing of the crude synthesis gas. According to the prior art, in order to avoid any reduction in hydrogen or carbon monoxide yield as a result of this phenomenon, known as co-absorption, in a first regeneration step, the scrubbing agent streams laden with carbon dioxide or with carbon dioxide and sulphur components during gas scrubbing are, after prior chilling, in each case expanded in a separate expansion vessel to a pressure which is generally less than one third of the pressure at which the gas scrubbing is carried out. Under the temperature and pressure conditions prevailing in the expansion vessels, the co-absorbed substances are largely converted into the gas phase, while the unwanted components scrubbed from the crude synthesis gas largely remain dissolved in the scrubbing agent. The gas phases from the two expansion vessels which, in addition to hydrogen and carbon monoxide, also respectively contain carbon dioxide or carbon dioxide and sulphur components, are then compressed and recirculated into the crude synthesis gas to be fractionated upstream of the gas scrubber. While an elevated product yield is indeed achieved in this manner, the costs which in particular arise for the necessary compression of the gas phases to be recirculated do, however, have a negative impact on the economic viability of the method.